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@battermann battermann/io.fsx
Last active Jul 3, 2018

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IO Monad in F#
[<AutoOpen>]
module IO =
type IO<'a> =
private
| Return of (unit -> 'a)
| Suspend of (unit -> IO<'a>)
let rec run x =
match x with
| Return v -> v()
| Suspend s -> s() |> run
[<RequireQualifiedAccess>]
module IOMonad =
let rtrn x = Return (fun () -> x)
let bind f io = Suspend (fun () -> f(io |> run))
let map f io = bind (f >> rtrn) io
let wrap r = Return r
type IOBuilder() =
member this.Bind(x, f) = IOMonad.bind f x
member this.Return(x) = IOMonad.rtrn x
member this.ReturnFrom(io): IO<_> = io
let io = new IOBuilder()
[<AutoOpen>]
module MaybeIO =
type MaybeIO<'a> = IO<Option<'a>>
let run (x: MaybeIO<'a>): 'a option = IO.run x
[<RequireQualifiedAccess>]
module MaybeIOMonad =
let rtrn x: MaybeIO<'a> = IOMonad.rtrn (Some x)
let bind f io =
IOMonad.bind (function
| Some x -> f x
| None -> IOMonad.rtrn(None)) io
let map f io = bind (f >> rtrn) io
let liftIO io = IOMonad.map Some io
type MaybeIOBuilder() =
member this.Bind(x, f) = MaybeIOMonad.bind f x
member this.Return(x) = MaybeIOMonad.rtrn x
member this.ReturnFrom(e): MaybeIO<_> = e
let maybeIo = new MaybeIOBuilder()
let readLine = IOMonad.wrap (fun () -> stdin.ReadLine())
let print x = IOMonad.wrap (fun () -> printfn "%A" x)
let foo = io {
let! cs1 = readLine
let! cs2 = readLine
let x = cs1 + cs2
let!_ = print x
return x
}
io {
let! x = foo |> IOMonad.map (fun (s:string) -> s.ToUpper())
let! _ = print x
return ()
} |> IO.run
let tryParse str =
match System.Int32.TryParse(str) with
| (true,int) -> Some(int)
| _ -> None
let readInt = readLine |> IOMonad.map tryParse
let maybePrint = print >> MaybeIOMonad.liftIO
let bar = maybeIo {
let! x = readInt
let! y = io { return 20 } |> MaybeIOMonad.liftIO
let result = x + y
let! _ = maybePrint result
return result
}
bar |> MaybeIO.run
@amieres

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amieres commented Jun 18, 2018

why not use do! instead of let! _ =?

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